1. Field of the Invention
The present invention relates to an electrically powered vehicle equipped with a power storage system for supplying an electric power to a drive source, and particularly to a structure for keeping a high estimation accuracy of a state of charge of the power storage system.
2. Description of the Background Art
Recently, hybrid vehicles and electric vehicles that are driven by a drive power of an electric motor have attracted attention as vehicles developed in view of the environment. These electrically powered vehicles are equipped with power storage systems for supplying electric powers to drive sources, i.e., the electric motors, respectively. Discharging and charging of the power storage system are performed in view of, e.g., a state of charge (which may be simply referred to as an “SOC” hereinafter) of the power storage system. Overcharge and overdischarge of the power storage system can be avoided by keeping the SOC within an appropriate range.
A battery assembly has been widely used as the power storage system. The battery assembly is formed of a plurality of battery modules (i.e., components thereof) connected together in series, and each battery module has a plurality of cells each formed of, e.g., a nickel hydrogen battery. The charge and discharge of the battery assembly is controlled by sensing battery voltages, e.g., of a plurality of battery blocks and arithmetically estimating the SOC from the sensed battery voltages.
However, when the battery characteristics deteriorate due to a long-term use of the battery assembly, variations occur in capacity between the battery blocks. For example, Japanese Patent Laying-Open No. 2005-166362 has disclosed a control system of a hybrid vehicle that can accurately grasp the variations in SOC that occurred between a plurality of cells in a battery module.
According to this system, it is determined whether a state of charge of the battery assembly has met conditions under which a memory effect is liable to occur. When the conditions are met, deep charge and discharge are executed multiple times to erase the memory effect of the battery assembly, and then processing is performed to sense the variations in SOC that occurred in the cells of the battery modules forming the battery assembly. Thereby, the system can accurately sense the variations in characteristic of the battery assembly.
However, as the method of sensing the variations in the battery assembly, Japanese Patent Laying-Open No. 2005-166362 has disclosed merely a method in which the battery assembly is discharged with a constant current, an SOC value is measured when the voltage variations of the battery assembly become large, and it is determined based on the SOC value whether the variations in SOC are present in the battery modules or not. It has not sufficiently disclosed a method of estimating the SOC of each battery module.
More specifically, when the battery characteristics deteriorate, the estimation accuracy for the SOC lowers. Particularly, the charge/discharge power of the electrically powered vehicle is extremely larger in quantity than that of consumer products, and the quantity of changes in environment temperature thereof is also large. Therefore, a remarkable difference occurs between the arithmetically estimated SOC and the actual SOC, resulting in a problem that sufficient estimation accuracy cannot be obtained. However, Japanese Patent Laying-Open No. 2005-166362 described above has not disclosed countermeasures against the above problem.